1. Technical Field of the Invention
The present invention is related to a novel micro-electro-mechanical systems (MEMS) torsional drive that is capable of tilting suspended structure such as a micro-mirror for steering light beams in three-dimensional analog fashion, which is suitable for high port count optical switches.
2. Description of the Related Art
Commercial MEMS-based all-optical switches are based on one fundamental principle and two well-understood approaches. The principle is simple: The switch routes photons from one fiber-optic cable to another. The routing is accomplished by steering the light through a collimating lens, reflecting it off a movable mirror, and redirecting the light back into one of N possible output ports. The two basic design approaches for translating this principle into optical switches are a two-dimensional (2-D) or digital approach (N2 architecture) and a three-dimensional (3-D) or analog approach (2N architecture).
The 2-D digital approach is so-called because the micro-mirrors and fibers are arranged in a planar fashion, and the mirrors can only be in either of two known positions (on or off) at any given time. In this approach, an array of MEMS micro-mirrors is used to connect N input fibers to N output fibers. This is called an N2 architecture, because it uses N2 individual mirrors. It is the optical equivalent of a cross-bar switch. For example, an 8×8 2-D switch uses 64 mirrors. A big advantage of this approach is that it requires only simple controls, essentially consisting of very simple, transistor-transistor-logic (TTL) drivers and associated electronic upconverters to provide the required voltage levels at each MEMS micro-mirror. Apart from a robust product line of N×N switches, including 4×4, 8×8, 16×16, and 32×32 ports, which use an input and an output fiber port, the 2-D planar approach supports the introduction of a third and fourth fiber port to a basic N×N switch. That permits dynamic add/drop functionality, arrays of 1×N switches in a single package, and customized mirror configurations on the chip. These features allow an array of mirrors to replace yesterday's cumbersome, expensive, custom discrete switch integrations with small, hermetically packaged, robust, custom switching configurations. Although the simple 2-D design is inherently flexible, the greatest challenge in this approach lies in scaling switching to very high port counts. As port count doubles, the distance the light must travel through free space is also doubled, and the diameter of the light beam grows, placing tighter constraints on collimator performance and mirror-alignment tolerance. Because of the length of the travel path for the signal, as well as the angle tolerance and angle uniformity required on the MEMS mirror itself, 32 ports are currently considered by most vendors a top-end size for a single-chip solution in 2-D technology. This is not to say that a 2-D approach is limited to 32 ports. On the contrary, there are architectures, including the well-known Clos approach that cascades smaller 2-D switches into a multistage architecture scalable to hundreds by hundreds of ports. An example is Siemens's Transexpress MODIF optical service node. Another benefit of the 2-D approach is the ability to move rapidly from development to high-volume manufacturing, while maintaining the optical performance of a hand-built component, coupled with the reliability and cost-effectiveness of a mass-produced product.
In many respects, the 3-D analog or beam-steering approach is actually very similar to the 2-D approach. It uses the same principle of moving a mirror to redirect light. The 3-D approach results in a 2N architecture, because two arrays of N mirrors each are used to connect N input to N output fibers. But in this approach, each mirror has multiple possible positions—at least N positions. This approach is much less constrained by the scaling distance of light propagation as the port count grows. Such architectures can scale to thousands by thousands of ports with low loss (potentially 6 dB or less) and high uniformity. These advantages come at a price, however, because the micro-mirror must have multiple possible positions, a sophisticated analog-driving scheme is implemented to ensure that the mirrors are in the correct positions at all times. Although MEMS technology can produce 2N 3-D mirror arrays with impressive stability and repeatability by using a simple open-loop driving scheme, closing the loop with active feedback controls is fundamental to achieving the long-term stability required in carrier-class deployment of an all-optical cross-connect. Using a closed-loop control scheme implies that monitoring the beam positions must be implemented in conjunction with computation resources for the active feedback loop and very-linear high-voltage drivers.
The MEMS technology that is used by most optical switch vendors is polysilicon-based surface micromachining technology, due to its demonstrated capability to fabricate sophisticated micromachines that optical devices tend to be. As mentioned before, this technology requires also expensive capital equipment and infracture that only a few large companies can afford. Most vendors developing MEMS-based switches typically outsource the manufacture of the actual structure to a foundry and focus their efforts on creating the subsystem in which they fit. However, after a few years of intense effort in optical switches development, it becomes clear that polysilicon is not a good choice for optical switches due to its inconsistent properties such as intrinsic residual stress that are detrimental to optical structures. Thus port count in 2D arrays is limited to 32×32 (OMM white paper), and high-volume production of such switches has not been established or demonstrated. More and more vendors are moving to single crystal material in developing their mirrors for optical switches, including Lucent technologies, Inc., Calient Networks, IMMI, and Texas Instruments using bulk micromachining technologies, in part or as a whole. These mirrors have less tendency to warp and are easier to make. However, they have not reached sophistication level and manufacturability that have been demonstrated with surface micromachining technology. They also tend to be large which makes it more difficult to grow the port count.
A useful cross-connect switch fabric should handle up to 1280 cross-connect (XC) channels (ports) at up to 3.2 Terabits/sec total capacity. It should be strictly non-blocking, growable to large size (>1024×1024), and polarization independent, should have <5 dB insertion loss, <1 ms switch time, wavelength independent, and <40 dB cross talk, and it must be manufactured at a low cost.
Although tilting micro-mirrors fabricated by micromachining techniques is an obvious candidate for making XC fabric due to their advantages in cross-talk, polarization- and wavelength-independence and bit-rate transparency, which prompted many companies to adopt this technique, implementing them in large array format is met with obstacles. The low insertion loss requirement is more difficult to meet for larger switches because it increases rapidly with the distance of light path, which is directly proportional to size of a MEMS optical switch. Such loss can be introduced by mirror angle divergence from 90 degrees, mirror angle uniformity across the array and travel distance variations along non-uniform path lengths, warp in the mirrors and/or support frames, Gaussian beam propagation, and divergence due to the collimating lenses.
Thus MEMS vendors have difficulties in growing the 2D arrays much larger than 32×32 ports as reported by OMM, who is resorting to the 3D architecture for larger switches, which shifts the task of aligning the light beam from simple digital setting to a subsystem that each mirror is controlled by analog circuits with closed-loop feedback, and multiple LEDs and detectors. Such subsystems are very complex and bulky, if a 72×72 switch developed by by Astarte, Inc. (recently acquired by Tellium), which measured 2 meters on a side, is an indication.
However, the polysilicon-based micromachining technology, because their actuators are made very large, compared to the size of the mirrors, for overcoming restorative force of the springs on the mirrors, improve switching speed to below 1 ms, and reduce yield loss due to poor optical alignment, thin film stress, and defects. Thus larger size switch is resorting to “3D” steerable mirror arrays, which was demonstrated by Lucent in 1992, but has not been shown to be production worthy. This type of optical switch, although requires much fewer micro-mirrors, are relatively large and difficult to make and assemble, its switching speed is slowed, and the cross connects are not non-blocking, which increases cost of ownership at the system or subsystem level.
As the port count grows, it becomes more and more difficult to route conductive lines from the edges of the chip to the center switches. A 32×32 array requires at least 16 conductive lines to run through at least one of the edge switches. This increases the chip size significantly and can become a limiting factor in growing the port count. This is even more challenging in the case of 3D arrays where every mirror has a control circuit with closed-loop feedback.
An important issue that most optical switch vendors have failed to address is the integration of electrical circuitry that drives or actuates the tilt of the micromirrors. This lack of integration strategy stems from the fact that the lateral, side-by-side integration of microcircuits used in actuators system-on-a-chip approach to optical switches where integration of mirror-controlling circuitry or even microprocessors are integrated on the same MEMS chip.
Micromachined optical switches use tilting or movable mirrors to direct light beams. The mirrors are manipulated with attached microactuators to initiate and control the movement. Perhaps the most popular microactuator is so called comb drive (FIG. 1a), which consists of a movable comb and a fixed comb as shown in FIG. 1. When a voltage is applied between the two combs, the movable comb is attracted to the fixed comb. The position of the movable comb is thus be controlled by the voltage. The comb drive, like most others, produce lateral displacement instead of rotational displacement or torque. Two actuators are reported in the literature that produces rotational movement. First one is a parallel plate actuator (FIG. 1b), which simply places two electrodes under two sides of a hinged metal plate and applies a voltage on the electrodes to tilt the hinged metal plate. A modification of this actuator uses electromagnets to provide the torsional force. A second torsional actuator (J. A. Yeh, et. al. J. Microelectromechanical systems, Vol. 8 (4), 456–465, 1999) consists of hinged polysilicon comb and bulk silicon comb to form an asymmetric combdrive where the polysilicon comb is offset from the bulk silicon comb.
The polysilicon comb is attracted to the bulk silicon comb with applied voltage by fringe field, which generates a force that is very weak at the beginning and increases rapidly when the gap closes until the two combs merge, at which point the torque diminishes. The amount of tilt is proportional to the thickness of the polysilicon layer, which is quite small. Thus the drive was combined with the parallel plate torsional actuator to provide larger tilt. Even so, the amount of tilt is limited to less than 10 degrees and the voltage required is large, more than 50 volts. Such voltage is too high for regular CMOS integrated circuits to provide, thus a discrete driver has to be used.